1. Field of the Invention
The present invention relates to an electrophotographic-type image forming apparatus, which is applicable to a copier, printer, facsimile machine or any composite apparatus thereof and, more specifically, to an image forming apparatus of the type which uses mono-component nonmagnetic toner as a developer for image formation. Still more specifically, the invention relates to an image forming apparatus which is capable of producing images with consistent quality without being influenced by variations in toner carry quantity and chargeability by the application of a predetermined bias voltage to a toner layer forming blade.
2. Description of the Related Art
Electrophotography has been used extensively in a copier, a laser beam printer, etc. as a technology for printing images on a sheet of plain paper as image recording medium. Electrophotographic development can be roughly grouped into wet developing and dry developing, and generally the latter dry method is more popularly used. The dry development can be further grouped into two processes according to the type of developer or development agent to be used. One developer is two-component developer and the other is mono-component or mono-component developer, and the development using the latter mono-component developer is prevailing predominantly as the developing method mainly for small-size printers or copiers. Mono-component developer contains no carrier such as iron or ferrite powder and the developing apparatus using mono-component developer can be constructed simpler than that using two-component developer, thus offering advantages in terms of compactness and cost over the two-component counterpart.
Thus, mono-component development uses mono-component nonmagnetic developer for image formation. As a method of mono-component developing, impression development is known to those skilled in the art, in which the electrostatic latent image on a photosensitive drum and toner particles or toner carrier are brought into contact with each other such that the relative peripheral speeds therebetween are substantially zero. Such mono-component developing method is disclosed in U.S. Pat. No. 3,152,012, U.S. Pat. No. 3,731,148, Japanese Patent Application KOKAI Publication No. 47-13088 and Japanese Patent Application KOKAI Publication No. 47-13089. This mono-component developing method has many advantages; for example, it can dispense with carrier or magnetic material and, therefore, the developing apparatus and hence the image forming apparatus operable according to this method can be constructed simpler in structure and smaller in size and also it makes easier to use color toners.
As compared with the two-component development, however, the mono-component development is disadvantageous in terms of toner chargeability and toner carriability (or toner carry quantity) during operation immediately after a start-up of the image forming apparatus because the mono-component toner as the developer has no carrier which serves to charge the toner and to carry the toner onto the developing roller. Improvement of toner chargeability has been provided, for example, by Japanese Patent Application KOKAI Publication No. 6-130790 or Japanese Patent Application KOKAI Publication No. 2000-221776, which each proposes changing the material for a toner charging blade from plastic to metal and also applying a bias voltage to that toner charging blade (referred to as xe2x80x9cblade biasing methodxe2x80x9d).
According to the proposed blade biasing method, the effect of charge injection, as well as that of triboelectrification, are available, so that consistent toner charging can be accomplished. However, in this blade biasing method, a difference in toner chargeability is recognized between the leading end portion of an image and the remaining portion thereof, when the developing apparatus is used constantly and when the apparatus is used after a start-up following an idle or unused state thereof for a while. To be more specific, fogging toner appears increasingly in the background area in the above former case, while irregular image density in the solid area occurs in the latter case.
The above-cited KOKAI Publication No. 2000-221776 also discloses a technology of controllably changing the bias voltage for application to the toner layer forming blade in dependence on a change in cohesion of the mono-component toner which occurs due to toner deterioration with age or by a change of ambient humidity. According to this technology, an optimum toner layer can be formed on the developing roller by changing the bias voltage across the toner layer forming blade according to a change in the toner cohesion, with the result that good image formation can be maintained successfully. However, a problem of inconsistent image quality remains unsolved because the toner chargeability varies with an increasing number of prints.
The present invention has been made in view of the above-described problems. It is an object of the present invention to provide an image forming apparatus for using mono-component nonmagnetic toner, the image forming apparatus being capable of forming images with consistent quality without being influenced by variations in toner carry quantity and chargeability. More specifically, bias voltages are applied to a toner feed roller, developing roller and blade through a toner feed bias supply, development bias supply and blade bias supply, respectively. In this regard, the blade bias voltage is increased by a predetermined magnitude after a release of sleep mode of the fixing device until a predetermined number of image recording media have been imaged. Additionally, the blade bias voltage is decreased for a period of time corresponding to the first turn of the developing roller and increased for a period of time corresponding to the second or more turns thereof. In this manner, the blade bias voltage and the toner feed bias voltage are controllably changeable with a life or an increasing number of image formations.
In order to achieve the above objects, according to one embodiment of the present invention, an image forming apparatus is provided with a photosensitive member for forming thereon an electrostatic latent image, a developing roller for supplying mono-component nonmagnetic toner to visualize the latent image on the photosensitive member, a toner feed roller for feeding the toner to the developing roller, a blade for forming a layer of toner on the developing roller, a fixing device for fixing the visualized image on an image recording medium, and a blade bias voltage controllable means for applying a bias voltage to the blade and controllably changing the blade bias voltage at a time when a predetermined number of image recording media have been imaged after a start of operation of the fixing device following an idle state thereof continued for a predetermined period of time or longer.
According to one aspect of the present invention, the blade bias voltage is higher during the period of time before the predetermined number image recording media imaged than that after this period of time.
According to another aspect of the present invention, the idle state of the fixing device is accomplished by sleep mode setting thereof and the start of operation of the fixing device is effected by terminating the sleep mode setting.
According to another aspect of the present invention, a life-dependent blade bias voltage controllable means is provided for controllably changing a bias voltage for application to the toner feed roller, as well as the bias voltage for application to the blade, in response to a life or an increase in the number of image recording media formed with images.
According to another aspect of the present invention, the bias voltage for application to the blade is increased with an increase in the bias voltage for application to the toner feed roller.
According to another aspect of the present invention, the absolute value of the bias voltage for application to the blade is lower than that of the bias voltage for application to the developing roller.
According to another aspect of the present invention, the life-dependent blade bias voltage controllable means is operable to controllably change the bias voltage for application to the blade in response to the number of turns of the developing roller.
According to another aspect of the present invention, the life-dependent blade bias voltage controllable means is operable to detect a change in physical characteristics of the toner feed roller based on the value of an electrical resistance thereof and to controllably change the bias voltage for application to the blade based on a change in the electrical resistance value.
According to another aspect of the present invention, the life-dependent blade bias voltage controllable means is operable to controllably change the bias voltage for application to the toner feed roller and the bias voltage for application to the blade on the basis of an accumulated running distance of the photosensitive member.
According to another embodiment of the present invention, an image forming apparatus is provided with a photosensitive member for forming thereon an electrostatic latent image, a developing roller for supplying mono-component nonmagnetic toner to visualize the latent image on the photosensitive member, a toner feed roller for feeding the toner to the developing roller, a blade for forming a layer of toner on the developing roller, a fixing device for fixing the visualized image on an image recording medium, and a blade bias voltage controllable means for controllably changing a bias voltage for application to the blade at a time when the developing roller has made a predetermined number of turns after a start of operation of the fixing device following an idle state thereof continued for a predetermined period of time or longer.
According to one aspect of the present invention, the predetermined number of turns is one turn.
According to another aspect of the present invention, the bias voltage for application to the blade is higher during the period of time before the developing roller has made the predetermined number of turns than that applied after this period of time.
According to another aspect of the present invention, means for detecting the running distance of the photosensitive member and determining the number of turns of the developing roller based on the detected running distance is provided, wherein the blade bias voltage controllable means is operable to controllably change the blade bias voltage based on the determined number of turns.
According to yet another embodiment of the present invention, an image forming apparatus is provided with a photosensitive member for forming thereon an electrostatic latent image, a developing roller for supplying mono-component nonmagnetic toner to visualize the latent image on the photosensitive member, a toner feed roller for feeding the toner to the developing roller, a blade for forming a layer of toner on the developing roller, a fixing device for fixing the visualized image on an image recording medium, and blade bias voltage controllable means for applying a bias voltage to the blade, controllably changing the bias voltage for application to the blade at a time when a predetermined number of image recording media have been imaged after a start of operation of the fixing device following an idle state thereof continued for a predetermined period of time or longer, and controllably changing the bias voltage for application to the blade about at a time when the developing roller has made a predetermined number of turns after a start of operation of the fixing device following an idle state thereof continued for a predetermined period of time or longer.
According to one aspect of the present invention, the blade bias voltage controllable means is operable to increase the bias voltage for application to the blade after the developing roller has made the predetermined number of turns and also to decrease the bias voltage for application to the blade after the predetermined number of image recording media has been formed with image.
According to another aspect of the present invention, the bias voltage for application to the toner feed roller is controllably changed with an increase in the number of prints and the bias voltage for application to the blade is also controllably changed with an increase in the number of prints, so that the toner charge quantity can be stabilized and image quality can be stabilized accordingly.
According to another aspect of the present invention, the blade bias voltage, as well as the toner feed roller bias voltage, is increased in response to a life, so that the toner charge quantity can be stabilized, which helps to reduce fogging on the photosensitive member.
According to another aspect of the present invention, in view of the phenomenon that leakage occurs due to the difference between the voltage applied to the developing roller and that applied to the blade when the toner carry quantity is reduced, the absolute value of the bias voltage for application to the blade (e.g. xe2x88x92350 V) is set lower than that of the bias voltage for application to the developing roller (e.g. xe2x88x92500 V). By so doing, leakage can be successfully prevented.
According to another aspect of the present invention, by establishing a relationship between the toner feed bias voltage and the developing bias voltage as |toner feed bias voltage|xe2x89xa7|developing bias voltage|, the toner supplied to the developing roller from the toner feed roller is subjected to triboelectric charging and, therefore, the toner is carried to the blade under the influence of electrostatic force, physical force and potential difference. The common voltage source for supplying the same bias voltage to the toner feed roller and to the developing roller helps simplify the image forming apparatus and its relevant peripheral circuits, and the controlling operation of the apparatus.
According to another aspect of the present invention, the blade bias voltage is increased by a predetermined magnitude at early stage of image forming operation, so that good image with consistent quality with little fogging can be produced.
A difference in charge quantity occurs due to rotation of the developing roller immediately after a start-up of the image forming operation, resulting in irregular image density. However, according to an aspect of the present invention, the blade bias voltage is controllably changed during the early period of rotation of the development roller so that the charge quantity difference is limited to realize consistent image density.
According to one aspect of the present invention, the blade bias voltage is so controlled that the blade bias voltage is changed during the early period of operation immediately after releasing the sleep mode setting of the fixing device, as well as changing the bias voltage during early rotation of the developing roller. This makes it possible to perform stabilized image formation from the early period of operation and even with the number of prints being increased.
Furthermore, irregularity in image density may occur due to a charge quantity difference occurring between the first turn and the subsequent second turn of the developing roller. However, according to one aspect of the present invention, the blade bias voltage for the first turn of the developing roller is decreased and the voltage is increased for the second and the following turns thereof so that the charge quantity difference can be limited to realize consistent image density.
The required difference in the bias voltages for application to the developing roller and the blade should differ in dependence on the number of prints made. As a method of changing the blade bias voltage in the first turn and the second and the following turns of the developing roller, therefore, it may be so controlled that the blade bias voltage is changed when the number of prints made which is represented by the accumulated running distance of the photosensitive drum has reached a predetermined value.
Alternatively, according to one aspect of the present invention, the blade biasing means can change the blade bias voltage in accordance with a value of voltage applied to the toner feed roller which may be detected by using a conductive brush or thin metal plate disposed in contact with the toner roller feeding roller surface.
Furthermore, according to one aspect of the present invention, the running distance of the photosensitive drum can be calculated by counting the number of rotations of the photosensitive drum by means of an encoder. This can eliminate a contact device such a conductive brush as described above, which may cause damage to the toner feed roller.